1. Field of the Invention
The present invention relates to plastics optical waveguides, more particularly to a plate plastics optical waveguide useful as an optical component, such as an optical coupler or multiplexer for optical communication or image transmission.
2. Description of Prior Art
As a base material for optical components or optical fibers, generally inorganic substances, such as silica glass and multicomponent glass, are used because their propagation loss is low and their transmission band is wide.
On the other hand, optical components using plastics as a base material have been developed. Plastics optical materials have good fabrication flexibility and controllability of optical properties compared with inorganic optical materials and, and therefore, much attention has been paid thereto with an expectation that optical waveguides having relatively good characteristics could be produced with ease using them. Representative techniques for producing plate plastics optical waveguides include selective photopolymerization and utilization of a photosensitive resin.
The selective photopolymerization method is a technique within which monomers contained in a polymer are selectively polymerized to change the refractive index to make a pattern-like optical waveguide. More particularly, at first a mask having a predetermined pattern is mounted on a polymer sheet or substrate composed of a transparent polymer such as polycarbonate which contains a low refractive index monomer, such as methyl acrylate, and the sheet or substrate are irradiated with ultraviolet rays through the mask to selectively polymerize the low refractive index monomer in accordance with the pattern. The photopolymerized portion of the polymer has a lower refractive index than the polymer matrix. Then the polymer sheet is heated in vacuum to remove unreacted monomers which remain in areas not exposed to ultraviolet rays. As a result, the unexposed portions of the polymer consist of the high refractive index polymer alone. Thus, a patterned sheet can be obtained in which a high refractive index portion serving as a core is formed in accordance with a predetermined pattern. Finally, the patterned sheet is sandwiched by a clad composed of a low refractive index polymer to produce an objective optical part.
On the other hand, according to the method which uses a photosensitive polymer, a photosensitive polymer is pattern-wise exposed to a light to cause crosslinking selectively, followed by development to remove unexposed portions to obtain a core pattern. More particularly, at first a polymer serving as a clad is coated on a substrate by dipping or spin coating. A polymer, such as polyurethane, containing a photosensitive crosslinking agent is coated on the clad in the same manner as above. Then, the coated polymer is irradiated with ultraviolet rays through a patterened mask to selectively crosslink the polymer. Next, the substrate is immersed in a solvent to remove unexposed portions to obtain a core having a predetermined pattern. Finally, a clad material is provided on the core by dipping, spin coating or laminating to form a clad and, thus, an objective optical part is produced.
In order to obtain practically useful optical parts which show a low optical loss, it is required that a film for transmitting a light is of good quality and patterns formed by microprocessing have a high reliability. That is, it is preferred that the material of the film itself show an optical loss which is as low as possible, and the thickness and refractive index of the film be controlled with high precision. Further, smoothness of side walls of the core, and dimension stability and reproducibility are important factors for the microprocessing of the waveguide fabrication.
In the case where a plastics material is used, optical parts produced by the selective photopolymerization method and the method using a photosensitive polymer, respectively, show a relatively low optical loss at shorter wavelengths (0.48 to 1.1 .mu.m). However, they show a high optical loss, as high as 0.5 to 10 dB/cm, in the infrared region (1.3 to 1.55 .mu.m), utilized in optical communication at present, because of higher harmonics of infrared vibrational absorptions due to the carbon-to-hydrogen bonds constituting the plastics, and therefore they are unsuitable for practical use.
From the standpoint of fabrication flexibility of optical waveguides, both methods are simpler and easier than methods employed in the production of glass optical waveguides. However, the selective photopolymerization method has some problems, such as, that the content of the monomer varies depending on the conditions of evaporation of the solvent which causes subtle fluctuation of the refractive index difference. On the other hand, the method using a photosensitive polymer has problem that resolution is poor and that protrusions and depressions tend to occur on the surface of the optical waveguide due to swelling upon development. These are reasons why conventional plastics optical waveguides show high optical losses.